In the chemical process and petroleum refining industries, maintenance and capital improvements of process plants normally occur at planned intervals, often three to seven years apart in the case of large petroleum refineries. Even if scheduled annually or biannually, cleaning is attempted when the operating unit is completely shut down and every aspect of its operation inspected and maintained. Operating units in hydrocarbon processing plants are made up of various types of vessels; i.e., tanks, heat exchanges, distillation columns, heaters, reactors and the like in fluid communication with each other. Characteristically, in refining and petrochemical operations, a feed, such as crude oil, is introduced to one end of the plant, with hydrocarbon product streams being removed at the other end, either to storage or to facilities for transportation to market. Of course, any similar process flow-stream equipment is also cleaned. This massive maintenance effort on each unit is called a turnaround and maximum speed in returning the unit to operation is essential to the plant economics, since the overriding cost of such an operation, even though itself expensive, is the time during which this operating unit is not producing refined product which can be sold. The speed at which the jobs are done are, of course, tempered with the necessity that safety of the workers be maintained throughout the operation.
Hydrocarbon processing equipment is designed for operation with various hydrocarbon feedstreams for a certain period of time to normally concur with the time lapse between scheduled turnarounds. For instance, a heat exchanger may be over designed so that, as its efficiency falls off due to fouling and contamination from operation, it passes through an optimum level of heat transfer efficiency to a level of toleration which diminishes as the time for the turnaround procedure approaches. It is not uncommon that, during operation of hydrocarbon processing plants, particularly crude oil heaters in refineries which operate in parallel with each other, some particular units will become less efficient at a much faster rate than others. For example, the efficiency of a single crude oil heater in a process stream can cause an overall decrease in the operational capacity of the entire refinery, thus causing a considerable depression of the economics of the refinery. The choice becomes whether to keep operating until a scheduled turnaround or to shut down the entire refinery for a long period of time in order to bring the overall efficiency back to a satisfactory level. Such a work stoppage would result in premature expense to maintain some of the equipment merely to bring one or two pieces of equipment up to par.
Normally, a turnaround operation for a large hydrocarbon processing plant may take three weeks to greater than a month of intensive round-the-clock operations. While the copending applications describe some of the advantages in time-saving through the practice of certain related inventions, the only alternative heretofore practiced in connection with the cleaning of these particular units has been to shut them down and then attempt to remove fouling through the injection of steam into the process equipment or unit. Initial injection of steam serves to remove hydrocarbons and volatile materials from the interior of the equipment. However, continued injection of high temperature steam often serves only to bake a scab of hard coke or scale on the walls of hydrocarbon processing units. Once this occurs, the only alternative is to disassemble the equipment and to proceed with cleaning of the parts in more conventional ways such as through hydroblasting with a high-pressure stream of water. A build up of carbon on heat exchanger tubing can cause hot spots to develop in the equipment resulting in erratic operation of the processing unit.
The present cleaning practice, depending upon the equipment, uses a light hydrocarbon solvent to initially remove heavy oils and tars, followed by injection of steam for a period of time until monitoring devices indicate that no dangerous gases remain which present an explosive hazard to workers who must work in this environment. This period of preliminary steam cleaning is called a "steamout" and has been known to last for days, even weeks on occasion. Process equipment was also washed with water to remove contaminants where applicable, and often, both steaming and water washing is involved in the degassing of a vessel to make it possible for humans to safety enter to inspect and repair. Frequently, acids or detergents were also used to remove stubborn deposits. These steps often take many hours, even days, to accomplish--days of lost production.
Of particular importance to worker safety is, in addition to elimination of explosive gases, the removal of benzene and other volatile organic carbon components, many of which are known carcinogens, from the process vessels prior to worker entry, if required. It has been long recognized that chronic exposure by humans to benzene at high levels in the chemical and petrochemical work place leads to bone marrow depression, aplastic anemia, and leukemia. Although absorption of benzene across the skin as a vapor or in aqueous media can occur, benzene toxicity in process systems is most frequently caused by inhalation of benzene that has managed to escape removal. Present government safety standards for eight hour work days are set at 1.0 ppm (average) benzene. The National Institute For Occupational Safety And Health (NIOSH) has recommended (1989) an occupational long term exposure limit in air of 0.1 ppm benzene. That is not good enough, since workmen employed by turnaround service companies are continually exposed to the interior of process vessels. Worker safety regulations now limit such exposure. One major goal of the present invention, therefore, is to provide a process for process equipment decontamination which exceeds this standard--in fact, which approaches, if not meets, 0 ppm of benzene. Benzene is often found collecting in head space of a vessel or trapped beneath scale or other contaminants anywhere along the proces flow stream--in piping, in valves, and in pumps, as well as towers, reactors, tanks and heat exchanges--only to seep out from the interstices of contamination deposits at a later time when cleaning had been thought to be completed.
In this era of quality management, when a particular piece of process equipment has lost its efficiency and requires cleaning in order to return the entire operating unit to useful efficiency, a procedure is needed for quickly accomplishing such cleaning and return to service through the removal of scales, absorbed hydrocarbons, cokes and other contamination from equipment and heat exchange surfaces without disturbing the entire unit when possible and certainly without requiring a prolonged shut down. Previously, such shut downs could last for weeks while extensive steaming and contamination removal continued during a steamout operation. The procedures required for such a cleaning, until now, are substantially described above. Where a single piece of process equipment rather than the entire unit is involved, it is necessary to unbolt flanges and install blinds to isolate the equipment while steam is introduced. Previously, when steam was used, venting to the atmosphere was allowed. Venting is no longer permissible under responsible environmental management, yet the steamout is strongly extended technology.
Even though there are many conflicting problems which converge on cleaning process equipment, the overriding requirements are still speed with safety, and the practice of this invention accomplishes these while providing avenues for a more effective protection of the environment. This invention allows almost pristine cleaning of process equipment--from the crude oil feed pump to product storage tank and any step along the way--without time-honored steamout in terms of hours, rather than days or weeks.